Chocolate chip cookie dough is an example of a

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Chocolate chip cookie dough is an example of a heterogeneous mixture . It is easy to see that it is a mixture because the chocolate chips are definitely different from the surrounding dough. It is heterogeneous because it is difficult to guarantee that the last scoop you place on the cookie sheet will have exactly the same number of chocolate chips as the first scoop did. Chemical vs Physical Properties How do we differentiate a sample of matter from another? When we compare samples of matter, we look at the properties or characteristics that distinguish them from one another. The properties may be chemical properties or physical properties. Chemical properties describe the composition (what is it made of) of a sample of matter and the way it may change or react with another material to form new substances (chemical reaction). Examples of chemical properties include elemental composition (for example, water is made of 11.2% hydrogen and 88.8% oxygen), flammability (whether something will burn), or inertness (lack of reactivity). Chemical changes are changes that result in formation of completely different substances. Examples of chemical changes include formation of rust or burning gasoline in a car engine. Once a chemical change occurs, it is usually not easy to return to the original material. Physical properties describe characteristics that can be determined without changing the composition or chemical identity of the substance. Examples are boiling point, density, color, physical state, hardness or softness, etc. It is possible to melt ice to form water, boil the water to form steam, condense the steam back to liquid water, and freeze the water into ice. In this case the water never changes its composition, it just changes its physical form. Another example would be dissolving salt in water. The salt seems to “disappear” but if the water is allowed to evaporate, the salt can be recovered, virtually unchanged. Page 2 of 3
Separation of Mixtures A mixture can be separated based on differences in the physical properties of the substances in the mixture. Any difference in physical properties can be the basis for a method of separation. For example, liquids that boil at different temperatures can be separated from each other by distillation. The liquid with the lower boiling point will evaporate more easily, resulting in separation. A method that we will study later in the course is paper chromatography. In paper chromatography, materials can be separated based on their affinity for being attracted to the cellulose molecules in paper compared to their solubility in a solvent. Law of Conservation of Mass Remember that a scientific law is a brief statement or mathematical equation summarizing a large body of data or observation or phenomenon. The Law of Conservation of Mass says that in a chemical reaction, matter is neither created nor destroyed. This law was first proposed by Antoine Lavoisier, who did many experiments in which he found that the masses of all the products in a chemical reaction equaled the total masses of all the reactants. This leads to the observation that

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